1. Field of the Invention
The present invention relates to brushless DC motor drives of the type configured for receiving electrical energy from a source. More particularly, the invention relates to a brushless DC motor drive suitable for use in the vehicular applications and the like, which is adapted to maintain accurate speed control of a brushless DC motor despite variations in the ground potential from the source. The invention also relates to a drive of this type which is adapted to draw a minimal amount of electrical energy when the associated motor is not being driven.
2. Description of the Related Art
Various motor configurations are known and are currently in use for driving equipment, such as fans and blowers in systems powered by a direct current source. Such applications often arise in vehicular systems, such as air conditioners, refrigeration units and the like provided on automobiles, buses, trucks, trailers, recreational vehicles and so forth. Many such applications employ conventional DC motors including a commutator and brushes. The rotational speed of such motors may be adjusted and controlled by control of the voltage applied to the motor. Another family of motors, often referred to as brushless DC motors, do not include a comutator and brushes, and are driven at speeds that are a function of the frequency or duty cycle of pulsed drive energy applied to the motor. In applications where the power source is a direct current system, the motor drive typically converts the direct current energy to pulse energy having a frequency or duty cycle which corresponds to the desired output speed of the motor.
Conventional brush-type DC motors offer a number of advantages, including simplicity and low cost. However, such motors suffer from serious drawbacks in a number of applications. For example, due to their inherent design, conventional brush type DC motors may have an extremely limited life (e.g. 2500 to 3000 hours). In applications where the motors are driven for extended periods each day, this limited life may result in frequent servicing of the systems in which the motors are installed, typically for replacement of the motors themselves.
Another drawback of conventional brush-type DC motors is their tendency to experience speed fluctuations due to variations in the voltage applied to the motors. Such variations may occur for an individual motor during different periods of operation of a vehicle, for example while climbing an incline, accelerating, idling and so forth. Also, voltage variations may occur between individual motors coupled to a common direct current source due to voltage drops in conductors connected between the source and the motors. Hence, in systems including a number of direct current motors, each motor may receive different supply voltages depending upon their location and the length of conductor between the source and the motors. Such voltage variations ultimately result in a lack of uniformity and precision in the speed control of the motors.
Conventional brush-type DC systems employ relays and contactors which physically interrupt electrical current carrying paths to the motors when they are not being driven. However, such relays and contactors add significantly to the size and cost of the motor package and drive system.
While brushless DC motors, driven by microprocessor-based drive circuitry may overcome certain of the drawbacks of brush-type motors, they have not conventionally been employed in many vehicular applications. Such drive circuitry may include closed-loop velocity control adapted to maintain output speed of a motor based upon a velocity command signal from a operator-adjustable device such as a potentiometer, or from an automatic controller. However, where voltage variations from the direct current source occur in the vehicles, even though the velocity command signals may not be affected, undesirable speed fluctuations may occur despite the closed-loop control circuitry, in much the same way as described above for conventional brush-type DC motors. Moreover, brushless DC motor drive circuits do not adequately avoid the problem of current draw during periods when the motor is not being driven. In particular, microprocessor-based brushless DC motor drive circuitry typically continues to draw undesirable current at levels sufficient to potentially drain the power source even when the motor velocity command signal is substantially zero.
There is need, therefore, for an improved brushless DC motor drive that is relatively insensitive to variations in ground potential from a power source, resulting both from changes in the load on the power source and from variations in the length of conductors linking the power source to individual motors in a system. Moreover, there is a need for an improved brushless DC drive system which avoids the problem of energy consumption due to current draw when the motor is not being driven.